The invention relates to an X-ray rotary anode comprising a carrier body of graphite and a target layer of tungsten or a tungsten alloy, a silicon-carbide layer being present between the carrier body and the target layer.
Such X-ray rotary anodes are used in X-ray tubes, in particular X-ray tubes for medical purposes. In the X-ray tubes, electrons of high energy originating from a cathode are launched onto the target layer of the rotary anode. When the electrons reach the target layer only a small part of the energy is released in the form of X-rays; the greater part (approximately 99%) is converted into heat. Since there is a vacuum in the X-ray tube, the dissipation of heat takes place mainly by radiation. Graphite is a material having a high heat-emission coefficient. Moreover, its specific mass is low relative to other customary carrier materials such as Mo or Mo-containing alloys. A low specific mass enables a high speed of the rotary anode, thus permitting an increase of the thermal load.
An X-ray rotary anode of the type mentioned in the opening paragraph is known from French Patent Application FR 2593325. The X-ray rotary anode described in this document comprises a carrier body of graphite, a target layer of tungsten or a tungsten alloy and an intermediate layer of, for example, rhenium or silicon carbide. Such intermediate layers enhance the adhesion between the target layer and the carrier body and reduce the diffusion of carbon from the graphite to the tungsten layer.
To increase the emission of heat by thermal radiation it is desirable to increase the operating temperature of the X-ray rotary anode from, at present, approximately 1400.degree. C. to approximately 1600.degree. C. Since the radiation energy delivered is proportional to the fourth power of the absolute temperature of a radiating body, the increase in temperature means that the output of thermal radiation energy is doubled. A disadvantage of the known X-ray rotary anode is that at such high operating temperatures carbon originating from the silicon carbide intermediate layer diffuses to the tungsten layer and forms tungsten carbides. At such high operating temperatures, a rhenium intermediate layer does not sufficiently preclude the diffusion of carbon from the graphite carrier body to the tungsten layer, so that tungsten carbides are still formed. Such tungsten carbides are brittle and cause mechanical stresses between the intermediate layer and the tungsten target layer. Delamination between the tungsten target layer and the intermediate layer takes place owing to large variations in temperature, thereby causing the target layer to insufficiently contact the graphite carrier body through the intermediate layer. The temperature of the target layer then rises in an uncontrolled manner, as a result of which the target layer becomes integrally detached and/or melts.